12V / 49 CFM: The SPL measurement says everything you need to know about
how the fan sounds at 12V: It's LOUD! Oh, and the cooling performance was pretty
good too, although nothing we haven't seen before.

9V / 24 CFM: At 9V, the airflow dropped by half, the load temperature
increased by 4°C, and the noise level dropped 16 [email protected] That's a pretty
significant difference folks. The noise level at 9V was still far from silent,
but it's quiet enough for use in an office environment or anywhere with a lot
of background noise. It came in just below the 30 [email protected] mark that we consider
the maximum amount of noise permissible from a "quiet" system.

Subjectively, the quality of the noise was poor. The dominant sound was
a harsh whine, with some buzz and a lot of resonance in the background. Turbulence noise was not an issue.

Cooling performance was still good enough for most systems, although overclockers
with the latest Pentium chips will probably want to look elsewhere.

8V / 15 CFM: 8V isn't one of our usual test points, but we included
it because it represents the best tradeoff between noise and performance for
this heatsink/fan combo. At this level, the noise dropped to 22 [email protected], which
is good enough for most quiet systems. The dominant noise was a resonant buzz,
although there was still a fair amount of whine.

The main source of noise was no longer the fan itself, but the plastic duct
that amplified the fan noise, especially the low frequency buzz. Without the duct attached, the buzz became quieter and
the whine dropped in both pitch and volume. Although the measured noise level
did not drop significantly without the duct in place, the subjective difference
was quite marked. The soft hum that the fan produces without
the duct, was tolerable, but the resonant buzz with the duct
was intolerable..

Cooling performance at this level falls into the category of "good enough".
High end systems may struggle to keep cool, but low and medium powered systems
should be fine.

7V / 5 CFM: At 7V, the noise level dropped enough that I had to struggle
to hear it. Once again, the main source of noise was the resonant buzz that
was made so much worse by the duct. I doubt I could have heard it if the duct
was removed.

Unfortunately, the cooling performance was so poor that it's basically unusable
at this level: The load temperature was 45°C above ambient, which could
easily be 30°C or more inside a computer case. Very few processors can withstand
load temperatures of 75°C for long.

5V / ?? CFM: Just for fun, the fan voltage was dropped to 5V, enough
that the noise level was below ambient and effectively silent. In fact, even
our lab microphone, placed at a distance of 3 inches, was unable to pick up
a signal. According to our measurements, running the fan at 5V had no effect:
It produced no noise, pushed no air, and provided no cooling. Not surprisingly,
the temperature rose quickly and the CPU soon started throttling.

The Nexus fan proved to be a better match for the Vapochill Micro than any
of the stock fans thanks to its superior airflow to noise ratio. It sounded smoother, and exhibited no low buzz. The duct did affect the noise of
the Nexus by increasing the motor hum a bit.

Cooling performance was acceptable when the fan voltage was between 9V and
12V, which is quiet enough for most systems. Below 9V, cooling performance was
marginal, probably unacceptable for most users.

The Vapochill Micro performed poorly with low airflow. The combination of
closely spaced fins and a duct that is partially open at the bottom means that
a lot of pressure is needed to force enough air through the heatsink. In a low
airflow situation there is very little pressure, so the air just takes the route
with the least impedance: Out the bottom of the duct without ever passing through
the fins of the heatsink.

LOOK MA, NO DUCT!

There are two counts against the plastic duct:

It is highly resonant and worsens the quality of noise produced by the fan.

It allows air from the fan to escape before it ever passes through the fins
of the heatsink. The effect is particularly bad when there is not much airflow

In view of this, a quick test was run without the duct in place. The reference Nexus fan was hung against the side of the the heatsink with crossed
fingers. This arrangement worked well enough for testing.

The test confirmed that the duct was hurting the cooling performance, but the
difference in load temperature was a paltry 3°C.
The measured noise level with the Nexus fan didn't change, but it was already low enough. Subjectively, there seemed to be a little less resonance,
but, like the difference in temperature, it wasn't a big change.

At higher fan speeds, the acoustic
benefit probably makes removing it worthwhile, but there doesn't seem to be
a huge thermal difference. The poor cooling performance with low airflow seems
to be caused by more than just the duct, so the only reason to remove the duct
is for acoustic reasons.